A conventional electron-detector comprises a scintillator plate having a scintillator material wherein the electron beams, the intensities of which are to be detected, are directed onto the scintillator plate. This scintillator material emits light beams caused by incident electrons. Light optics are provided in order to image this scintillator plate into a region where light receiving areas of a light detector are provided. By detecting light incident onto the light receiving areas, it is possible to deduce the intensities of the electron beams directed onto the scintillator plate.
It has been shown that the light intensities detected by the light detector are unexpectedly low, in particular after a longer period of operation of the electron-detector.
The present invention has been made taking the above considerations into account.